Soybean saponins (hereinafter, simply noted as “saponins”) are a generic name of saponins contained in starting soybeans and are contained in soybean hypocotyls in an amount of about 2 to 4% by weight. Saponins are classified into group A saponin, i.e., bisdesmoside saponin, wherein an aglycone skeleton is “soyasapogenol A” and sugar chains are attached to C-3 position and C-22 position of the aglycone through ether bonds; group B saponin, i.e., monodesmoside saponin, wherein an aglycone skeleton is “soyasapogenol B” and a sugar chain is attached to C-3 position of the aglycone through ether bond; and the like. In addition, saponin, wherein a moiety of a sugar chain is acetylated is also reported (Kitagawa et al, Chem. Phrm. Bull, 33, (1985)).
 R1R2R3Group A saponinSugar ChainSugar Chain—OHGroup B saponinSugar Chain—OH—OH
On the other hand, isoflavones are contained in starting soybeans as trace components other than saponins, and contained in soybean hypocotyls in an amount of about 1 to 2% by weight. Isoflavones are generic name for isoflavone aglycone whose aglycone skeleton is daidzein, glycitein or genistein; isoflavone glycoside, wherein a sugar chain is attached at position 6 of the isoflavone aglycone; and further acetyl isoflavone glycoside or malonyl isoflavone glycoside, wherein its sugar chain has an acetyl group (—COCH3) or a malonyl group (—COCH2COOH) as a functional group.
Saponins are said to have a variety of physiological activities such as anti-obesity activity, antioxidant activity, and an immunity activation activity. Then, when a material containing saponins at a high concentration is available, the material can be utilized by simply adding it to various foods and drinks.
Conventionally, in order to obtain a material containing saponins at a high concentration from starting soybeans such as whole soybeans, defatted soybeans, soybean hypocotyls, and the like, it has been known that appropriate separation of saponins from isoflavones is important (JP 62-5917 A and JP 61-129134 A). Hence, as described in these gazettes, these processes require carrying out purification by gel filtration such as LH-20, or the like, or partition chromatography after carrying out first stage production steps involving extraction with a solvent such as a lower alcohol or acetone, adsorption of the resultant extract with a synthetic adsorption resin such as HP-20 and XAD-2 or activated carbon, washing the absorbed extract with a low concentration of a lower alcohol (10 to 40% by volume) and then elution of a fraction containing saponins with a high concentration of a lower alcohol (70 to 80% by volume).
However, in order to obtain a highly pure saponin fraction in the above-described processes, the final purifying step by means of gel filtration or partition chromatography is required and a multi-stage purifying step is essential. Hence, a yield of saponins is as low as about 1 to 2%. Indeed, for example, the yield of saponins disclosed in Example 1 of JP 62-5917 A from starting soybeans is 1.38%. This is problematic from the viewpoints of facility costs and production efficiency for industrially putting such a process into practice. On the other hand, in order to extract saponins with a solvent at a high yield, conventionally, the extraction is carried out at a high temperature. However, even when such extraction is carried out, a material containing saponins which satisfies requirements of high purity and high yield without complicated operation cannot be obtained.
Thus, an object of the present invention is to facilitate separation of saponins from isoflavones extracted from starting soybeans to obtain a highly pure saponin-containing material at a high yield on an industrial scale.